A dynamic voicing filter for an electronic organ

ABSTRACT

An electric organ having conventional voicing circuits is also supplied with an auxiliary dynamic voicing filter, controlled by the expression pedal of the organ. The voicing filter is an active RC filter, having a feedback loop of controllable gain, that gain determining the peak frequency of the filter. A diode gate in the feedback loop is controlled by a control voltage established as a function of position of the expression pedal of the organ.

United States Patent inventor David A. Bunger Cincinnati, Ohio Appl. No.1.918 Filed Dec. 31, 1969 Division of Ser. No. 723,685, Apr. 28, 1968Pat. No. 3,519,720 Patented Sept. 14, 1971 Assignee D. H. BaldwinCompany Cincinnati, Ohio A DYNAMIC VOICING FILTER FOR AN ELECTRONICORGAN 14 Claims, 3 Drawing Figs.

US. Cl 84/l.l9, 84/DIG. 9, 84/D1G. 10

Int. Cl G10h 1/02 Field of Search 84/1 .01

1.l1,1.12,1.19,1.2,1.21,1.22,l.24,DIG.9, DlG. 10; 330/31 [56] ReferencesCited UNITED STATES PATENTS 2,647,173 7/1953 Beurtheret 84/D1G. 102,568,797 9/1951 Eland 84/D1G. 10 3,166,622 1/1965 Neustadt 84/].013,255,296 6/1966 Peterson 8411.24 3,316,341 4/1967 Peterson 84/1 .243,486,126 12/1969 Chin et a1. 330/31 X Primary Examiner-Milton O.Hirshfield Assistant Examiner-Stanley .1. Witkowski Attorneys-W. H.Breunig and Hurvitz, Rose & Greene ABSTRACT: An electric organ havingconventional voicing circuits is also supplied with an auxiliary dynamicvoicing filter, controlled by the expression pedal of the organ. The v0-icing filter is an active RC filter, having a feedback loop ofcontrollable gain, that gain determining the peak frequency of thefilter. A diode gate in the feedback loop is controlled by a controlvoltage established as a function of position of the expression pedal ofthe organ.

TAB 51D.

were" PATENTEB sen 4 new SHEET 1 [IF 2 Enw 205mm xm XQOBPNZ xuamcwwu vINVENTOR DAVID A BUNGER ATTORNEYS A DYNAMIC VOICING FILTER FOR ANELECTRONIC ORGAN This application is a division of my application, Ser.No. 723,685, filed Apr. 28, 1968, now U.S. Pat. No. 3,519,720 andentitled Organ Having Dynamically Variable Timbre with TransistorizedPlayer Controlled Dynamic Filter."

BACKGROUND OF THE INVENTION Prior U.S. patents relating to the subjectof this invention are: Peterson, No. 3,255,296; Peterson, No. 3,316,341;and Neustadt, No. 3,166,622.

These patents involve dynamic modification of the output of tonegenerators of an electric organ. The present invention involves use ofan active tunable RC filter of novel configuration, as a device fordynamically modulating the timber of a tone, but in addition involvesthe use of the in any event available expression pedal of an organ tocontrol timber modulation, while that pedal remains available to theorgan for expression control. Thereby the total number of controls whichthe player is called on to manipulate, in practicing the presentinvention, is not increased over the number required in the conventionalorgan, yet the total number of musical effects available to theinstrument being played is increased.

SUMMARY OF THE INVENTION An electric organ conventionally includes atone generator having discrete complex tone sources which can beselectively called forth by key switches. The complex tones, i.e. tonescontaining many partials, are passed through voicing filters which areselectively placed in circuit by voicing tabs. The voicing filtersdetermine the relative amplitudes of the partials, for each tone, whichare passed on to amplifiers and eventually to loudspeakers. Expressionis the term employed to indicate loudness of the radiated acousticsound, and in the conventional organ expression is controlled by a footpedal actuated by the right foot of the player.

According to the present invention, a dynamic voicing filter is-providedwhich is additional to and connected in parallel with the usual voicingfilter and which can be selectively operatively connected in the systemat will, either in place of or in addition to the usual voicing filters.The peaking frequency of the dynamic voicing filter is a function of theposition of the expression pedal. The dynamic voicing filter per serepresents circuitry and principles of operation which are novel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a graph depicting variousavailable response curves of a variable-timber-control filter, employedin the system of the invention;

FIG. 2 is a block diagram of a system of the invention: and FIG. 3 is aschematic circuit diagram of a dynamic filter of the system of FIG. 2,and of controls for the dynamic filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 2, is a signalgenerator which contains a signal source for each of the array ofcomplex tones required in an electronic organ. The separate tones arecalled forth by closing key switches'll, as by keys'or pedals (notshown) in the process of playing the organ. The tones called for by thekey switches 11 are collected on a bus 12, which leads to conventionalorgan voicing filters 13. The latter are conventionally selected byvoicing tabs, not separately shown. The outputs of the voicing filter orfilters selected by the voicing tabs proceed to a preamplifier 14,followed by an amplification stage 15. The latter drives a poweramplifier 16 which in turn drives a loudspeaker 17. A control voltagederived from a voltage divider l8 actuated by an expression pedal 19(FIG. 3) is applied to lead 20. That control voltage is applied as again-control signal to preamplifier 14, so that the loudness of soundradiated by the loudspeaker 17 is a function of the position ofband-pass filter which has an adjustable peaking frequency as a functionof control voltage, and a relatively low O which varies between valuesof about 2 and 5 as the control voltage varies. Peak frequencies mayvary from about 300 Hz. to about 1.5 kc., as the control voltage varies.The output of filter 30 proceeds via a tab switch 31 to preamplifierstage 15, following the preamplifier 14 which is subject to gaincontrol. Stage 14 may be incorporated in preamplifier 15. The expressionpedal thus simultaneously controls gain of the conventional organoutput, and tonal characteristics of a subsidiary output which itselfhas constant gain.

The dynamic filter 30 includes an input terminal 40 and an outputterminal 41. See FIG. 3. A first transistor amplifier stage 42 employinga transistor T, in an emitter follower configuration having circuitryarranged to provide a band-pass filter response having a peak at about1.5 kc. and drives the base of an output transistor T, arranged tooperate as a linear amplifier, and the collector of the latter iscoupled to output terminal 41.

The emitter follower T, also drives a feedback loop, including incascade a voltage-controllable diode gate 44, a phasereversing amplifier45 employing transistor T and an emitter follower stage 46 whichprovides a low resistance feedback point.

More specifically, the input terminal 40 is connected to the base of Tvia blocking capacitor C isolating resistance R,, filter capacitor C,. Abias circuit for the base of T, extends from a positive voltage supply48, to ground via resistances R A and R in series with each other. Thejunction of the latter resistances is connected to the base of T,. Asmall collector resistance 49 is provided and an emitter load,consisting of resistances 50 and 51 connected in series to ground, inthe order named. A resistance R, is connected between the emitter of T,and the junction of R, and C,. Circuit values are provided in thedrawings.

The gain of the amplifier 42 is a function of frequency, as follows:

liaise-swat on the assumption that the junction of R,, R, is at ACground potential, as is also the junction of R R,,. S is the usualsymbol for complex frequency.

The actual ground is provided by the emitter follower resistance R; of TThis accounts for the presence of terms R ,R, /R,+R, and R R,,/R,,+R,,.

Assuming zero feedback from T the characteristic of the amplifier isthat illustrated at B of FIG. I.

The AC output voltage available at point 60, at the junction ofresistances 50, 51, is applied through resistances 61, capacitor 62 tothe cathode of diode D,. That cathode is connected to ground through aresistance 63. The relative values of 61 and 63 essentially determinethe signal level transmitted to diode D,, the capacitor 62 introducinglow frequency rolloff and thus introducing minor filtering into thefeedback path which assists in controlling low frequency response. Theanode of d is connected to the anode of diode D in a backto-backconfiguration and the anode of D is connected to ground via resistance64. The anodes of D, and D, are connected to control lead 20, via R, towhich is applied control voltage decreasing as a function of extent ofdepression of expression pedal 19.

The cathode of D is connected to ground via a resistance 65, and througha coupling capacitor 66 to the base of transistor T Transistor T, has agrounded emitter and a collector load 70. A bias circuit and feedback isconnected from side of which communicates with the junction of R, and Cand Diodes D and D provide gating of signal through the feed back path.in absence of control voltage at the anodes of D D the gate representsan open circuit and therefore there is no feedback. The filtercharacteristic is then of type B, FIG. 1 of the accompanying drawings.

With no feedback the filter characteristic is determined by the valuesof C C and associated resistances, as seen from point G.

As control voltage to the gate is increased, the feedback loop becomesmore and more effective. This has the effect of increasing the effectivecapacitances of C and of C and of the associated resistances. If weconsider Here S is complex frequency. With feedback, the overall gain iswhere -K is gain of the feedback loop.

The net effect is to increase the last term of the denomina' tor of G bya factor of K, K depending on the impedance of the gate as acontrollable factor, an since the last term of the denominators Y and Tare the same, except for the factor K, the net effect of the feedbackloop is to increase the effective values of all frequency-determiningvalues of the filter in the same degree.

The mathematics of the system are approximate, so that the severalcurves of FIG. I are only approximately identical in shape, but therelative accuracy of the mathematics is attested to by the similarity ofshape of the several curves representing different values of gatingvoltage. Some deviation of the frequency response of the actual circuitfrom calculated response as a result of phase shift in the amplifier 45which was assumed to have 180 phase shift in the calculated expressions.The dots on curves A and 8 represent points 3 db. down, and indicatecircuit Q. Calculations of Q are not provided.

lclaim:

l. A tunable band-pass electronic music voicing system, including a tonesignal input terminal,

a transistor amplifier having an input control electrode, saidtransistor amplifier having an output circuit,

a first tone filter capacitor connected in series between said signalinput terminal and said control electrode,

means including a further amplifier connected in cascade with saidoutput circuit,

said further amplifier having a grounded resistive output circuit,

a second tone filter capacitor connected between a point located betweensaid terminal and said first capacitor and an ungrounded point of saidgrounded resistive output circuit,

said further amplifier being arranged to introduce only a single phasereversal between said terminal and said grounded output circuit, and

an output circuit means for said first mentioned transistor amplifier.

2. The combination according to claim 1, wherein said means including afurther amplifier further includes an AC gate connected in seriesbetween said output circuit of said transistor and an input circuit ofsaid further amplifier, said AC gate being responsible to a controlvoltage to vary its impedance.

3. The combination according to claim 2, wherein said load circuit meansincludes a further amplifier responsive to signal in said output circuitconnected to said first mentioned transistor amplifier, and

a loudspeaker driven from said further amplifier.

4. A tunable tone signal voicing filter having a band-passcharacteristic, comprising a first transistor amplifier having a baseelectrode, a collector electrode and an emitter electrode,

a first load resistance connected between said emitter electrode andground,

a signal input terminal,

a capacitive tone signal filter connected in series between said signalinput terminal and said base electrode,

a second feedback transistor amplifier including at least one furthertransistor having a base electrode, an emitter electrode and a collectorelectrode, said second transistor amplifier being arranged to introduceonly one phase reversal,

means driving said second transistor amplifier from said first loadresistance,

a second load resistance connected between the emitter of said secondtransistor and ground,

a capacitive tone signal filter feedback connection between anungrounded point of said second load resistance and a point between saidsignal input terminal and said capacitive tone signal filter,

a bias circuit for said first transistor including a resistanceconnected between said base of said first transistor and ground, and

a load device for said first transistor.

5. The combination according to claim 4, wherein said means includes avoltage-controlled gate connected between an ungrounded point of saidfirst load resistance and the input circuit of said second transistoramplifier.

6. The combination according to claim 5, wherein said load deviceincludes another amplifier responsive to tone signal across said firstload resistance, and a loudspeaker coupled to be driven by said anotheramplifier.

7. The combination according to claim 6, wherein said capacitive tonesignal filter and said capacitive-tone signal filter feedback connectionhave substantially equal capacitances.

8. A tone signal band-pass amplifier having adjustable peakingfrequencies comprising a first tone signal amplifier stage having afirst series capacitive tone filter in series with an input circuit ofsaid amplifier and having a first output circuit,

a negative feedback tone signal amplifier connected in cascade betweensaid output circuit and said input circuit, said negative feedback tonesignal amplifier having a further output circuit,

a second series tone signal capacitor connected from said further outputcircuit and via said first series capacitive tone filter in drivingrelation to said first tone signal amplifier, and

a variable voltage-responsive impedance between said first outputcircuit and a drive circuit for said negative feedback tone signalamplifier.

9. The combination according to claim 8, wherein said variable voltageresponsive impedance includes a voltage-responsive AC diode gate havingback-to-back diodes, and means for applying control voltage to thejunction of said gates.

10. The combination according to claim 9, wherein said diode gateincludes at least one low frequency rolloff capacitor connected inseries with said back-to-back diodes.

11. A dynamic band-pass tone signal filter having constant gain withvariable peaking, comprising connected a first transistor filter stagehaving a transistor in an emitter follower configuration and having aband-pass filter response, and r a feedback loop driven by saidtransistor, said feedback loop including a controllable diode gateexhibiting voltage-controllable impedance to said tone signal, saidfeedback loop further including a phase-reversing amplifier providing alow-resistance feedback point coupled to drive said first transistorfilter stage.

12. A variably peaking wideband signal band-pass filter comprising afirst transistor amplifier having an input circuit and an outputcircuit, and

a negative feedback path coupling said output circuit to said inputcircuit, said feedback path including a variable impedancevoltage-responsive diode gate capable of transferring said tone signal.

13. The combination according to claim 12, wherein said feedback pathfurther includes a series low frequency rolioff capacitor.

14. The combination according to claim 13, wherein said input circuitfurther includes a first series connected tonea

1. A tunable band-pass electronic music voicing system, including a tonesignal input terminal, a transistor amplifier having an input controlelectrode, said transistor amplifier having an output circuit, a firsttone filter capacitor connected in series between said signal inputterminal and said control electrode, means including a further amplifierconnected in cascade with said output circuit, said further amplifierhaving a grounded resistive output circuit, a second tone filtercapacitor connected between a point located between said terminal andsaid first capacitor and an ungrounded point of said grounded resistiveoutput circuit, said further amplifier being arranged to introduce onlya single phase reversal between said terminal and said grounded outputcircuit, and an output circuit means for said first mentioned transistoramplifier.
 2. The combination according to claim 1, wherein said meansincluding a further amplifier further includes an AC gate connected inseries between said output circuit of said transistor and an inputcircuit of said further amplifier, said AC gate being responsible to acontrol voltage to vary its impedance.
 3. The combination according toclaim 2, wherein said load circuit means includes a further amplifierresponsive to signal in said output circuit connected to said firstmentioned transistor amplifier, and a loudspeaker driven from saidfurther amplifier.
 4. A tunable tone signal voicing filter having aband-pass characteristic, comprising a first transistor amplifier havinga base electrode, a collector electrode and an emitter electrode, afirst load resistance connected between said emitter electrode andground, a signal input terminal, a capacitive tone signal filterconnected in series between said signal input terminal and said baseelectrode, a second feedback transistor amplifier including at least onefurther transistor having a base electrode, an emitter electrode and acollector electrode, said second transistor amplifier being arranged tointroduce only one phase reversal, means driving said second transistoramplifier from said first load resistance, a second load resistanceconnected between the emitter of said second transistor and ground, acapacitive tone signal filter feedback connection between an ungroundedpoint of said second load resistance and a point between said signalinput terminal and said capacitive tone signal filter, a bias circuitfor said first transistor including a resistance connected between saidbase of said first transistor and ground, and a load device for saidfirst transistor.
 5. The combination according to claim 4, wherein saidmeans includes a voltage-controlled gate connected between an ungroundedpoint of said first load resistance and the input circuit of said secondtransistor amplifier.
 6. The combination according to claim 5, whereinsaid load device includes another amplifier responsive to tone signalacross said first load resistance, and a loudspeaker coupled to bedriven by said another amplifier.
 7. The combination according to claim6, wherein said capacitive tone signal filter and said capacitive tonesignal filter feedback connection have substantially equal capacitances.8. A tone signal band-pass amplifier having adjustable peakingfrequencies comprising a first tone signal amplifier stage having afirst series capacitive tone filter in series with an input circuit ofsaid amplifier and having a first output circuit, a negative feedbacktone signal amplifier connected in cascade between said output circuitand said input circuit, said negative feedback tone signal amplifierhavinG a further output circuit, a second series tone signal capacitorconnected from said further output circuit and via said first seriescapacitive tone filter in driving relation to said first tone signalamplifier, and a variable voltage-responsive impedance connected betweensaid first output circuit and a drive circuit for said negative feedbacktone signal amplifier.
 9. The combination according to claim 8, whereinsaid variable voltage responsive impedance includes a voltage-responsiveAC diode gate having back-to-back diodes, and means for applying controlvoltage to the junction of said gates.
 10. The combination according toclaim 9, wherein said diode gate includes at least one low frequencyrolloff capacitor connected in series with said back-to-back diodes. 11.A dynamic band-pass tone signal filter having constant gain withvariable peaking, comprising a first transistor filter stage having atransistor in an emitter follower configuration and having a band-passfilter response, and a feedback loop driven by said transistor, saidfeedback loop including a controllable diode gate exhibitingvoltage-controllable impedance to said tone signal, said feedback loopfurther including a phase-reversing amplifier providing a low-resistancefeedback point coupled to drive said first transistor filter stage. 12.A variably peaking wideband signal band-pass filter comprising a firsttransistor amplifier having an input circuit and an output circuit, anda negative feedback path coupling said output circuit to said inputcircuit, said feedback path including a variable impedancevoltage-responsive diode gate capable of transferring said tone signal.13. The combination according to claim 12, wherein said feedback pathfurther includes a series low frequency rolloff capacitor.
 14. Thecombination according to claim 13, wherein said input circuit furtherincludes a first series connected tone-signal filter capacitor for saidtone signal and wherein said feedback path includes a transistoramplifier connected in cascade with said diode gate and providing alow-impedance coupling resistance, and a further series tone-signalfilter capacitor connected between said coupling circuit and via saidseries-connected tone-signal filter capacitor to complete said feedbackpath.